622 



ECOLOGY AND EVOLUTION 



sexes together and avoiding mismating. It 

 should be noted that such benefits accrue 

 for the species as a whole and not just for 

 the individual, thus indicating a principle 

 diflferent from Darwinian sexual selection 

 (p. 688). It is also necessary to emphasize 

 that sexual characters may be used for 

 threat or combat between males of the 

 same species (p. 691), especially in the 

 establishment of breeding or nesting terri- 

 tory. Here we have considered the isolating 

 eflFect, but it is obvious that other factors 

 may also play a role in the evolution of 

 the same character in the same species. 



Psychological isolation is important in 

 maintaining species distinction in sympatric 

 pairs or groups of species, but alone it 

 seldom if ever initiates speciation. 



PHYSIOLOGICAL ISOLATION 



The separation of psychological (p. 619) 

 and physiological isolation may be unwar- 

 ranted, but it is sometimes possible to dis- 

 tinguish between direct biochemical inter- 

 action and psychological sexual attraction 

 or repulsion. Physiological isolation also of- 

 ten merges with genetic isolation (p. 623) 

 (see Caspari, 1948). 



Patterson (1946, 1947) and Wheeler 

 (1947) show that the mucous membrane 

 of the vagina of many species of Drosophila 

 enlarges, shortly after the introduction of 

 semen, to three or four times its normal 

 virginal size. The semen without the 

 spermatozoa can bring about this reaction. 

 In intraspecies (homogamic) crosses the 

 vagina returns to its normal size in a few 

 hours, but in interspecies crosses it may be 

 enlarged for several days and undergo 

 changes that interfere with its normal 

 function. If a strong reaction occurs, the 

 females may not mate again. In extreme 

 cases eggs passing through the vagina may 

 disintegrate. Interference with fertilization 

 is the usual reaction. Patterson refers to 

 this physiological change as the insemina- 

 tion reaction. In the D. mulleri group of 

 Drosophila, the small number of hybrids 

 seems to be the result of this insemination 

 reaction. In interspecies copulation between 

 the two most closely related species, D. 

 mojavensis and D. arizonensis, the vagina 

 in the females of D. mojavensis recovered 

 sufficiently in some cases to allow the pro- 

 duction of hybrids. The species of the sub- 

 genus Sophophora do not show an insem- 



ination reaction even in homogamic mat 

 ings. The physiological isolation described 

 for some species of Drosophila would pre- 

 vent the exchange of genes, and mutations 

 afiFecting the protein composition of the se- 

 men could well initiate the division of a 

 species. 



Sonneborn (1941) has shown that varie- 

 ties of Paramecium aurelia with heritable 

 diflFerences in their reproductive and phys- 

 iological characteristics, but without mor- 

 phologic differences, do not interbreed. In 

 addition to complete sexual isolation be- 

 tween nonconjugating varieties, Sonneborn 

 and Dippell (1946) find isolation between 

 conjugating varieties caused by incomplete 

 sexual compatibility, hybrid inviability or 

 low viability, low viability of the F2 or 

 backcross generations, and an incompletely 

 understood selective elimination of hybrids. 



Paramecium bursaria is divided into 

 three noncrossing varieties in the United 

 States, and three additional varieties have 

 been reported from Russia (lennings and 

 Opitz, 1944) and Europe (Chen, 1946). 

 Reproductive isolation is maintained by 

 different surface chemical reactions of the 

 cell that prevent conjugation, or, in certain 

 mating types of variety II from the United 

 States and variety IV from Russia, conju- 

 gation may occur, but both members die 

 because of physiological incompatibility 

 (Chen, 1946a). Geographic or habitat 

 isolation mav also separate these varieties, 

 but, even when they occur together, phys- 

 iological isolation prevents the sharing of 

 any genetic characteristics that may have 

 arisen subsequent to their isolation. Such 

 physiological isolation may be closely akin 

 to physiological incompatibilitv between 

 spermatozoa and eggs with resultant infer- 

 tilitv (p. 623). Although these reproduc- 

 tively incompatible populations of Parame- 

 cium are classified as varieties of a species, 

 they conform to the definition of a true 

 species (p. 625), distinguished bv phys- 

 iologic rather than by morphologic char- 

 acters. 



Various classes and tvpes of intraspecies 

 reproductive incompatibility are summa- 

 rized and discussed bv Stout (1945). He 

 classifies incompatibilities such as those 

 found in Paramecium as diplont-diplont in- 

 compatibilities. Haphnt-haplont types are 

 found in the fungi in which reactions "oc- 

 cur in the interrelations between individ- 



